"Haunted" Temp gauge

96 Explorer XLT 4WD 4.0 pushrod V-6.....started to notice the temp gauge wasnt hardly moving much off the full COLD mark.....taking longer to get good heat from vent, etc. Thermostat stuck open is what I'm thinking.

SO....I get one, drain radiator, replace T-stat, fill radiator....fire it up.....wait til I see coolant circulating.....top off the radiator, put cap on and look at gauge. It sitting about 1/4 way between C and H....ok great problem solved.

BUT.....on the road test (9-10 miles)....gauge will climb WAY close to pegging hot, then drop back down to normal....climb again, and drop down. This happens at steady throttle and at idle. While it has not shown an overheat condition, the up-n-down gauge has me not trusting it. Got good heat, coolant level ok....so why the hinky gauge?? I'm perplexed and welcome ideas/thoughts. Thanks in advance.

I was thinking the same as XSleeper.
If I remember correctly, you can remove the rad cap and open the top on the overflow cap. Slowly squeeze and release the upper rad hose which should force the fluid to move around a bit. This can work the air bubble out.

Hmmm......30 + yrs of spinning wrenches and have never encountered this same scenario.

Having said THAT....the air bubble theory I have now heard from 3 separate forums. Is this a Ford thing?? All my years were spent in a Chevrolet/GM garage. But I will indeed try the "squeeze the top hose" approach. Recommend stone cold engine or one just cooled enough to get radiator cap off?

very likely air bubble. you have to bleed air out completely. there may be bleeder screw, normally in t-stat housing.
otherwise, if you have radiator cap, get some extras, start engine, and remove cap. have heat set to max. it will overflow some, until t-stat opens, then level will start dropping down, and you keep adding coolant until it's stable. close and refill expansion tank after engine cools, as it will suck coolant into the system.

hey, found this for you:
Ford specific

First a caution: NEVER ever think of releasing a radiator - coolant cap when the engine is hot, or even warm!!!

Burping: In order to what is sometimes referred to as 'burping' the cooling system - getting a big air bubble out of the closed cooling system, and IF the car has a radiator cap, you will actually need to have the car running with the cap off.

During the initial fill, fill the coolant reservoir bottle to the top of the cold fill range.

Set the temperature blend selector to the full warm position and the fan motor control to high for the front and rear heaters

Start the vehicle with the reservoir cap off and allow to idle until the thermostat opens
NOTE: the coolant level will drop quite a bit when the thermostat opens - that's most of the air bubble in the system being driven to the top of radiator and escaping. Without the air, there's more room for coolant.

Top off the coolant in the reservoir bottle to the top of the cold fill range,

Install and tighten the reservoir cap.

Increase the engine rpms to 3,000 for 15 seconds, and then return to idle for one minute.

Repeat the previous step four times or until heat is coming from the rear unit while the vehicle is idling.

Allow the vehicle to cool and top off the coolant in the reservoir to the cold fill range.

Note: if you find the heater going cool or cold when idling, that's a good indicator you might have to burp the system again. Equally likely just adding more coolant to the overflow reservoir will suffice too.

Did I mention to NEVER ever think of releasing a radiator cap when the engine is hot, or even warm? !!!

This is the best ever clue to burp the system and I still had problems with air bubbles. I finally got it to quit blowing cold air due to air in the cooling system.

1 - Do it like the person above advises

2- Open up the radiator cap while it is still cold start the motor and when you see the coolant flowing after the thermostat opens. stick a long funnel into the filler neck because the fluid is going to overflow in a hurry.

3- The long funnel is going to capture the fluid and when it goes up your funnel then it's going to drain back into the radiator you may loose some fluid due to overflowing the funnel because it's going to belch about a half of a gallon or more - so be prepared to immediately pour some more antifreeze back down the funnel to keep the radiator full and keep the air out when the water pump moves the fluid back into the motor. At all costs keep replenishing the fluid when it belches air and fluid out of the radiator into your funnel or air will get back into the motor and will defeat your attempt to keep the air out.

4- You have to do this 2 times to rid the system of air before it works so wait for the thermostat to open again and repeat pouring fluid back into the system. Don't piddle around while doing this because once the water pump sucks air in the radiator because your not pouring the fluid back in fast enough and your not going to belch it.

Have a friend to help you get ready to pour antifreeze back into the radiator when it cycles. I had a catch pan under the car to catch the spilled fluid and reused it to pour it back in after the first belch so your friend can put that into a container to get ready to fill the funnel into the radiator again on the second belch.

5- no matter what - DON'T let the radiator level get low at any given time!!! keep pouring in the fluid fast as you can to keep the radiator completely full so it will have no air in it when the thermostat opens and cycles fluid through the water pump.

6- Wear safety glasses because fluid will spill and hit the engine belts while it's running and it will fling fluid everywhere.

It took me several attempts experimenting how to purge the system and this way I found definitely works for me and never had a problem since. I worked for Ford as a line mechanic for several years and this windstar is the meanest system I've ever purged. So if you ever have to change your antifreeze and drain the radiator and engine block this is the way I found to purge the system of air successfully... Thanks to the fellow above on his advise it helped out a lot.

I'm going to give you some more hint's that lead to this problem.
Look at the bottom side of your radiator cap you'll see a rubber washer mounted under the spring cap. If the rubber gets larger than the cup it's mounted on it will seal against the neck and won't let excess pressure escape from the cooling system effectively nor will it let fluid return into the radiator from the reseroir - so you'll need to buy a new one but WATCH-OUT even the parts stores will sell you a new one with the rubber sticking past the cup it's mounted on (The ones made in Mexico are the bad ones to buy) Just make sure the rubber is a little smaller than the circumference of the cup it's mounted to and you'll be alright... This applies to all vehicles and equipment new and old.

I've found parking your windstar on a kind of a steep hill with the drivers side higher than the passenger side will force the air in the heater core and the engine to release air in the system through the thermostat a little quicker and is more effective in purging the system...

Hmmm......30 + yrs of spinning wrenches and have never encountered this same scenario.

Having said THAT....the air bubble theory I have now heard from 3 separate forums. Is this a Ford thing?? All my years were spent in a Chevrolet/GM garage. But I will indeed try the "squeeze the top hose" approach. Recommend stone cold engine or one just cooled enough to get radiator cap off?

It's reasonably common with turbos or at least vehicles where the turbo is cooled with the coolant.
My Caliber SRT4 had this issue when I did the turbo swap.